Video signals are often corrupted by noise during the capture, storage and transmission steps. Noise levels are especially high when video is acquired during low-light conditions. The effect of the noise not only degrades the visual quality of the acquired video signal, but also renders encoding and compression of the video signal more difficult. Random noise does not compress well. Consequently, random noise may result in substantial bit rate overhead in the encoded video signal if it is to be compressed. Filtering of video signals is one method that may be used to reduce or remove noise from video signal. This filtering desirably enhances the video quality, and at the same time makes video data easier to encode. Pre-compression filtering of video data has captured more and more attention from the video compression industry. The design and implementation of such filtering often differentiates one product from another.
Consideration of both spatial and temporal characteristics of the video signal is desirable when designing video filters, such as a video pre-filter. In non-motion areas of the received video signal, it may be desirable to apply a temporal filter, while in areas with motion, applying a spatial filter may be desirable. The use of a temporal filter in area of frames with significant motion may cause the addition of motion blur. Using a spatial filter in non-motion areas of frames may lessen the noise reduction effect of a video pre-filter. Thus, designing a pre-filter that has both spatial and temporal filtering capabilities and that can dynamically adjust its spatial-temporal filtering characteristics to the received video content is desired.
In order to avoid motion blur, a motion adaptive spatio-temporal filter (MASTF) uses a motion-detection-based adaptive mechanism to smoothly switch between temporal filtering and spatial filtering of the video signal when motion is detected. The MASTF provides a good balance between performance and costs, which makes it attractive for use in many consumer electronics products; however, MASTF does not take full advantage of all of the temporal correlations that may exist in a video sequence. It only applies temporal filtering in areas of frames determined to be non-motion areas. As a result, the performance of MASTF in areas of frames in which motion is detected may not be as good as its performance in non-motion area.
Embodiments of the present invention include video filters that may improve both the quality of the resulting video and the compression rate when encoding the video signal. These are described below.